I'll leave the page to download the patch here:
https://novahq.net/files.php?ID=728&ID=728
When you open the page, click on the link of the title with black letters:
Doing this will download an .exe file which is actually the patch.

---

Once we have the downloaded file, we double click on it with the mouse. When you open it, you may see a window warning if you want the application to make changes to the device. Click on Yes and then this window will appear:

---

What you should do now is click on the down arrow. When you press it, this list will appear:

---

Of the 5 options to choose from, we select the one that says Delta Force 1 Window Border Fix.

---

Once selected, click on the option below: Install / Update Selected App Fix.
Once this is done, another window will appear in which it will tell you that the update was applied correctly.
With this, the .exe file called Df that appears in the Delta Force folder on steam, will be replaced. In this way, when you run the game, it will appear in fullscreen and not in the appearance of a windowed mode with borders.

Hope this guide has helped you a lot in correcting the fullscreen issue.
If you liked the guide, give it a positive assessment and share it with the community so that they can see that there is another solution to this problem.